A coal ash spill that blanketed residential neighborhoods and contaminated nearby rivers in Roane County, Tenn., earlier this week is more than three times larger than initially estimated, the Tennessee Valley Authority said on Thursday.

Coal ash, a byproduct of burning coal, contains toxic heavy metals like arsenic, lead and selenium that can cause cancer and neurological problems.

What may be the nation's largest spill of coal ash lay thick and largely untouched over hundreds of acres of land and waterways Wednesday after a dam broke this week, as officials and environmentalists argued over its potential toxicity.

Federal studies long have shown coal ash to contain significant quantities of heavy metals such as arsenic, lead and selenium, which can cause cancer and neurological problems. But with no official word on the dangers of the sludge in Tennessee, displaced residents spent Christmas Eve worried about their health and their property and wondering what to do.

The spill reignited a debate over whether the federal government should regulate coal ash as a hazardous material. Similar ponds and mounds of ash exist at hundreds of coal plants nationwide.

A coalition of environmental groups including Kentucky Waterways Alliance has sued the Interior Department and U.S. Environmental Protection Agency, seeking to overturn a new rule that will make it easier for mining companies to dump waste rock into streams.

The revisions, made final Dec. 12, will let mining companies disregard a 100-foot stream buffer zone if they are able to convince regulators that no other option was available and that they had taken steps to minimize harm to the environment.

"The notion that coal mining companies can dump their wastes in streams without degrading them is a fantasy that the Bush administration is now trying to write into law," said Judith Petersen, executive director of Kentucky Waterways Alliance.

More than 100 million people living in 46 metro areas are breathing air that has gotten too full of soot on some days, and now those cities have to clean up their air, the Environmental Protection Agency said Monday.

The EPA added 15 cities to the sooty air list, mostly in states not usually thought of as pollution-prone, such as Alaska, Utah, Idaho and Wisconsin. That's probably because of the prevalence of wood stoves in western and northern regions, a top EPA official said. But environmentalists said the EPA was only doing half its job on soot-laden areas, letting some southern cities with long-term soot problems _ such as Houston _ off the hook.

The U.S. Environmental Protection Agency routinely allows companies to keep new information about their chemicals secret, including compounds that have been shown to cause cancer and respiratory problems, the Journal Sentinel has found.

The newspaper examined more than 2,000 filings in the EPA's registry of dangerous chemicals for the past three years. In more than half the cases, the EPA agreed to keep the chemical name a secret. In hundreds of other cases, it allowed the company filing the report to keep its name and address confidential.

This is despite a federal law calling for public notice of any new information through the EPA's program monitoring chemicals that pose substantial risk. The whole idea of the program is to warn the public of newfound dangers.

The EPA's rules are supposed to allow confidentiality only "under very limited circumstances."